Following the discovery of carbon nanotubes in 1991, nanomaterials have attracted great research interest due to their unique properties that allow them to be ideal building blocks in important applications such as catalyst supports, absorbent, batteries, solar cells, capacitors, sensors, and polymer composite. Mixed metal oxide nanoparticles are particularly useful in applications such as forming stable electrocatalysts for oxygen evolution, oxygen reduction and nitrogen oxide reduction and as semiconductors for photoelectrochemical water splitting. Depending on the intended application, the mixed metal oxide may require a specific stable phase, such as being provided as a perovskite, pyrochlore, solid solution, or spinel.
Mixed metal oxides have traditionally been synthesized using a solid-state route that involves the mechanical milling of oxides, carbonates or nitrates. However, the solid-state route requires a long processing time and elevated temperatures and pressures to produce the mixed metal oxide. Wet chemical methods, such as sol-gel processes, co-precipitation, solvothermal techniques, and pyrolysis in air, have attempted to overcome the deficiencies of the solid-state route. However, these methods tend to be multi-step and laborious processes that require prolonged heat treatments and, therefore, produce low yields of product which are primarily restricted to equilibrium phases. Further, it can be difficult to control the size, composition, and phase of the mixed metal oxide nanoparticles. More recently methods for preparing mixed metal oxide nanoparticles have included flame spray pyrolysis (FSP) methods such as vapor-fed flame spray pyrolysis, liquid-fed flame spray pyrolysis, and liquid-fed flame assisted spray pyrolysis. But these FSP methods are limited to zirconium, aluminum, titanium, and silicon based oxides for preparation of the mixed metal oxide nanoparticles, and/or the method generates environmentally harmful gases, such as volatile chlorides.
Thus, there exists a need in the art for a more economical and efficient method of producing a variety of mixed-metal oxide nanoparticles of uniform size, novel composition and high-purity with predictable phases. Preferably, these mixed-metal oxide nanoparticles may comprise two, three, four, five, and potentially more metals.